Time

[Rule]

What is time?  Is it absolute?  What does "simultaneity" mean?  Is time travel possible?  Is time travel just "theoretically" possible but impractical to consider, or would it be easy to do?  Do velocities add -- for example, if a train is going 10 m/s wrt the ground, and a ball on the train is going 5 m/s wrt the train, is the ball going 15 m/s wrt the ground?  Does mass change with velocity?  If we were in a ship's cabin going at a constant velocity, would we be able to conduct any kind of physical experiment that would reveal we are moving?  For example, would the results of any experiment be different inside a ship's cabin than it would be on "static" ground?

[Eric]

What is time?  Is it absolute?

No, it's realitive.

Is time travel possible?

Yes, forward time travel is theoretically possible... or atleast it has yet to be disproven.

Do velocities add -- for example, if a train is going 10 m/s wrt the ground, and a ball on the train is going 5 m/s wrt the train, is the ball going 15 m/s wrt the ground?

No.

Does mass change with velocity?

Yes,  E=MC².

If we were in a ship's cabin going at a constant velocity, would we be able to conduct any kind of physical experiment that would reveal we are moving?

Yes, waves would be causing the ship to move/rock.

[Kaleeko]

Time is not absolute. If it were, anomolies like the "Twin Trip" would not occur. I'm not going to explain it because most people  know what it is, and I'm terrible at explaining things anyways.

But, because it's not absolute, time travel *is* possible; if one were to leave the Earth in the case of the twin trip, and return, it's very probable that they'd be much younger than the one who stayed behind on Earth. Also obvious. My main reason for replying though is, even if this method of time travel somehow became available to us, would we take advantage of it? Would it do us any good  to send people into "the future" when there isn't any way for them to return? This isn't a very academically based question, but I wonder what your opinion is.

[Rule]

Yes,  E=MC².

How is that equation proof that mass changes with velocity?  It looks like you've written mass as a constant there.

If we were in a ship's cabin going at a constant velocity, would we be able to conduct any kind of physical experiment that would reveal we are moving?

Yes, waves would be causing the ship to move/rock.

Very funny :P.

Would it do us any good  to send people into "the future" when there isn't any way for them to return? This isn't a very academically based question

It's an important question though.  It turns out relativity is an incredibly applied theory that does affect our everyday lives, and has a lot of potential to affect it more (in a very positive rather than an "indifferent" kind of way).  I'll chime in more soon. :)

[Sidoh]

What is time?

In the simplistic (and incompetent) way my teacher described it, time is "the ticking of clocks."

Is it absolute?

No.  Time intervals are relative to velocity and gravitational acceleration.

  What does "simultaneity" mean? 

When two events occur at the same time.  "Simultanious" is a relative term.  One oberver could accurately conclude that two events occured at the same time, but another observer at a different location (or under a different velocity/acceleration) might not agree.

Is time travel possible?

Yes.  Since time is relative to velocity, someone traveling at 99.99% the speed of light for a year could deaccelerate and come back to a world that had progressed much further than a year.  I've sort of forgotten the equations to figure out the exact time it would have been, but I'm sure I'll soon be reminded (college/you :P).

  Is time travel just "theoretically" possible but impractical to consider, or would it be easy to do?

At the moment, it's wholly impractical.  Rockets travel nowhere near to the speed of light.  I suppose if they traveled at top speed for a really, really long time the time descrepancy would grow, but that's impractical since its fuel is limited and the lives of the people opperating the rocket are limited.

When we discover a way to travel faster (near light speed), then yes, it'd be easy to do.  Of course, you'd have to allow a long time for acceleration...

I read something that if a craft were to undergo Earth's gravetational acceleration (9.8 m/s^2 in free space), it would take about a year to get to 90% the speed of light.  The memory is hazy, so it could probably use some correction.

  Do velocities add -- for example, if a train is going 10 m/s wrt the ground, and a ball on the train is going 5 m/s wrt the train, is the ball going 15 m/s wrt the ground?

Yes, unless you're dealing with light.

  Does mass change with velocity?

Yes.  Since energy and mass are equivilant (E=mc^2) and velocity carries energy, the higher the velocity of an object, the more energy it carries.  Since this is true, the amount of mass it carries also grows.

  If we were in a ship's cabin going at a constant velocity, would we be able to conduct any kind of physical experiment that would reveal we are moving?  For example, would the results of any experiment be different inside a ship's cabin than it would be on "static" ground?

No.

[AntiVirus]

Is time travel possible?

Yes, forward time travel is theoretically possible... or atleast it has yet to be disproven.

I read somewhere that they sent a particle through time. A very short amount of time, but they still did it.

[Sidoh]

I read somewhere that they sent a particle through time. A very short amount of time, but they still did it.

There are quite a few empirical proofs of relativity.  The way you worded it is very misleading, since it implies that the particle entered some sort of "void," "traveled through time" and then "exited the void."

There have been experiments with Muons (heavy electrons) that have suggested (more or less proven) that special relativity is correct.  When an Ozone molecule is broken by ultraviolet light (obviously a common occurrence in our atmosphere), it ejects a Muon at an ultra high speed (close to the speed of light).  Muons decay rapidly (a few nanoseconds).  If time were universal, it would be extremely easy to discover what point in spacetime the Muon would decay.  However, since special relativity states that time is relative, not universal, this is not the case.  Since the Muon travels far past where it would if time were universal and the equations provided by special relativity are able to accurately predict at which point in spacetime the Muon will decay traveling at its measured velocity, one can reasonably conclude that special relativity is true.

There have been similar experiments with radioactive particles.

[Eric]

  Does mass change with velocity?

Yes.  Since energy and mass are equivilant (E=mc^2) and velocity carries energy, the higher the velocity of an object, the more energy it carries.  Since this is true, the amount of mass it carries also grows.

Which is why Einstein concluded that it is impossible to accelerate any object with a mass to the speed of light: the mass would continue to increase to an almost infinite density as its velocity approached the speed of light and therefore making it impossible to accelerate further.

Quote from: Rule on Yesterday at 10:00:47 PM
  Do velocities add -- for example, if a train is going 10 m/s wrt the ground, and a ball on the train is going 5 m/s wrt the train, is the ball going 15 m/s wrt the ground?

Yes, unless you're dealing with light.

From who's point of reference?  To someone outside of the train, the ball would be moving at the same rate as the train (10 m/s).  To someone inside of the train, the ball would be moving at 5 m/s.

Composition of velocities - velocities (and speeds) do not simply 'add', for example if a rocket is moving at 2/3 the speed of light relative to an observer, and the rocket fires a missile at 2/3 of the speed of light relative to the rocket, the missile does not exceed the speed of light relative to the observer. (In the this example, the observer would see the missile travel with a speed of 12/13 the speed of light.)

[Sidoh]

Which is why Einstein concluded that it is impossible to accelerate any object with a mass to the speed of light: the mass would continue to increase to an almost infinite amount making it impossible to accelerate further.

Mmhm. :p

From who's point of reference?  To someone outside of the train, the ball would be moving at the same rate as the train (10 m/s).  To someone inside of the train, the ball would be moving at 5 m/s.

That wasn't my point.  Objects that can't travel at the speed of light can (but don't always) have additive velocities, if said velocity doesn't exceed the speed of light (I think, anyway. :\).

In example, a car traveling down a highway at a constant velocity can observe another car on the opposite side traveling at double its velocity relative to a still observer (ie, someone standing in the middle of the two lanes).

[Eric]

Which is why Einstein concluded that it is impossible to accelerate any object with a mass to the speed of light: the mass would continue to increase to an almost infinite amount making it impossible to accelerate further.

Mmhm. :p

From who's point of reference?  To someone outside of the train, the ball would be moving at the same rate as the train (10 m/s).  To someone inside of the train, the ball would be moving at 5 m/s.

That wasn't my point.  Objects that can't travel at the speed of light can (but don't always) have additive velocities, if said velocity doesn't exceed the speed of light (I think, anyway. :\).

In example, a car traveling down a highway at a constant velocity can observe another car on the opposite side traveling at double its velocity relative to a still observer (ie, someone standing in the middle of the two lanes).

ah ok.

[rabbit]

AV was right.  Two atomic clocks were synced up in a lab, and one was sent on a plane trip around the world.  When the two clocks were compared, the one which took the plane flight was ahead by a billion trillionth of a second (or something, I forget).

Anyway, time travel is possible.  Look at a watch and wait for it (the display) to change.  There, you've just gone into the future (which is now the present, but is now the past!).

If in a sealed container with no ways to observe the outside world, and which is moving at a constant velocity, there is no way to tell how fast you are going, though it is possible to tell time is moving (move your arm, etc...).

There is exact simultenaity (sp?).  It's something to do with boolean particle pairs.  I forget what exactly, but when one is in a certain state, its pair is in the opposite.  When one is flipped, the other simultaneously (sp?) flips, regardless of the distance between the two.

[Sidoh]

AV was right.  Two atomic clocks were synced up in a lab, and one was sent on a plane trip around the world.  When the two clocks were compared, the one which took the plane flight was ahead by a billion trillionth of a second (or something, I forget).

I didn't say he was wrong.  I said his wording was convoluted and misleading.

Thats said, this experiment really isn't the best proof of relativity, considering the margin of error is much, much smaller than it is with other experiments (like the one I explained).

Anyway, time travel is possible.  Look at a watch and wait for it (the display) to change.  There, you've just gone into the future (which is now the present, but is now the past!).

Smartass! ::)

If in a sealed container with no ways to observe the outside world, and which is moving at a constant velocity, there is no way to tell how fast you are going, though it is possible to tell time is moving (move your arm, etc...).

Being conscious is enough to determine that time is traveling.  If you're unable to determine that time is traveling, you're either:
  a) Dead or unconscious
  b) A photon or some other particle that can travel at c.

There is exact simultenaity (sp?).  It's something to do with boolean particle pairs.  I forget what exactly, but when one is in a certain state, its pair is in the opposite.  When one is flipped, the other simultaneously (sp?) flips, regardless of the distance between the two.

I don't really think that quantum entanglement much relation to simultaneity in the context Rule used it, but I could be wrong.

[Nate]

Yes Time Travel is possible.  Only problem is everything in the universe is going the same speed.

[Sidoh]

Yes Time Travel is possible.  Only problem is everything in the universe is going the same speed.

If that were true in every frame of referance, then time travel would not be possible. :\

[dark_drake]

If you were able to go faster than the speed of light, would you back in time? :-\